Aspirin, an important drug in the therapy of many connective tissue diseases including systemic lupus erythematosus, has been found (Seaman, W.E. and Plotz, P.H., Arth. Rheum. 19:155-160, 1976) to raise serum creatinine and BUN and lower creatinine clearance when administered to some patients with SLE as well as to some normal volunteers. Although this effect appears to be rapidly and completely reversible, if it is unrecognized it may falsely suggest deteriorating renal function due to activity of the underlying disease. Our studies have shown that this effect is related to renal blood flow and tubular function, and that it correlates with renal output of prostaglandin. Other drugs besides aspirin which inhibit prostaglandin synthetase also cause these effects.